Lysimachia japonica Thunb. var. minutissima Masam.

Family: Primulaceae
English: Loosestrife or Creeping Jenny (sp)
Japanese: ヒメコナスビ (姫小茄子) [Himé Konasubi]

Little yellow flowers that tend to follow human settlement. A dwarf wild variety is said to grow in Yakushima's subalpine regions, but it's questionable how much this variety has been contaminated with genes from the larger, more common species type.

References:
九州野山の花 片野田逸朗,2004：南方新社
米倉浩司・梶田忠 (2003-)　「BG Plants 和名－学名インデックス」（YList），http://ylist.info（ 2019年8月14日）